1. Field of Invention
This invention is directed to systems and methods for embedding visible watermarks.
2. Description of Related Art
Methods for protecting copyrightable works have evolved from a simple designation below the works to highly complex methods for embedding watermarks in the work. Watermarking can take two basic forms: visible and invisible. Examples of visible watermarks are the commonly-seen copyright logos or symbols that are generally affixed to the work before sales or distribution. Especially in the case of images, the presence of the watermark is very visible, and is generally difficult to remove without damaging the image. Generally speaking, visible watermarks do not harm the image, while providing a very robust form for encoding information in the image.
Invisible watermarks can encompass the standard and commonly used copyright logos or symbols, as well as company logos, serial numbers, or general identification marks and/or encrypted data. These invisible watermarks are embedded into the work in a way which is not generally discernible without the aid of visualization device such as a key or computer. Theoretically, these embedded images can be retrieved from the work at any time in the works"" history or from any other form or embodiment into which the work may have been translated. This allows the owner to track the work and clearly establish ownership rights when those rights are in dispute. Furthermore, since the embedded watermark image is essentially invisible to the unaided eye, the likelihood of tampering with or removal of the watermark is reduced.
Invisible watermarks, by definition, are essentially invisible to the unaided eye. Therefore, to retrieve such an invisible watermark, usually a hardware or a software technique needs to be applied to the watermarked image in order to retrieve the embedded information.
This invention provides systems and methods for embedding visible watermarks in images or documents.
This invention further provides systems and methods that allow visible watermarks to be embedded in images through a halftoning process.
This invention also provides systems and methods for modifying a halftone screen when the halftone screen contains the watermark to be embedded in a halftone image.
Embedding of visible watermarks is accomplished by altering a halftone screen prior to halftoning of an input image. Specifically, by modifying all threshold values within a specified area of halftone screen, and setting the gray-scale range to a constant value within this range, any existing halftone screen, e.g., clustered halftone screens or dispersed halftone screens, can be converted to a halftone screen that produces visible digital watermarks in the output halftoned image. The visible watermarks will appear in the output image as symbols defined by the specified area where the halftone screen was modified through the selected gray-scale range. Thus, the watermark is visually apparent within the output image and is very difficult to remove.
In particular, using an existing halftone screen, a portion of the screen is identified as an area which will correspond to the visible watermarks in the output image. The area can correspond to a letter, a word, a logo, a symbol, a signature, a pattern, a texture, or any other indicia a user desires to reproduce in the output image. In addition to selecting the area in which the visible watermarks will appear, a gray-scale range is selected such that the watermarks will only be shown when the values of the input-image fall into this range. For example, to convert an existing halftone screen to a screen for visible watermarks, the threshold values of the halftone screen within the specified area and within the selected gray-scale range are replaced by, for example, the upper limit of the selected range, for example in a 256 gray level image, 255, Those threshold values of the halftone screen within the specified area, that are not within the selected gray-scale range, are not changed. Therefore the specified area changes brightness but does not change to a solid, uniform output gray level. When the input gray values lie within the selected range, for example between 160 and 200, the halftone output by the modified screen inside the specified area will show no difference from the constant input equal to, for example, 161. Thus, the background, which is outside the specified area, varies as the halftone output by the unmodified screen. Therefore, the contrast between the inside and the outside areas makes the watermark visible.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.